Typically, they'll allow you to simply email the word doc. of the addendum to the Adcomm. You might email the Adcomm. and say attached is my (e.g. CF) Addendum. Please add it to my file, etc. List your LSAC Account Number as well. You might wait until your app goes complete so they actually have somewhere to put it when they receive it.

On applications without a slot to upload my addendum, I attached it to the end of one of the required documents, such as the PS or resume. I made sure it was clearly titled and started on a new page so it didn't appear that I had just tried to slip in an extra page.